In fast neutron nuclear reactors, the fuel assemblies constituting the core of the nuclear reactor disposed in the reactor vessel must be replaced after a certain length of service.
In this type of reactor, means are therefore disposed which allow extraction of irradiated assemblies from the reactor core and then from the reactor vessel so that they can be transferred to a storage and decontamination region or to a means of transport and temporary storage.
These extraction and transfer means generally comprise a transfer machine disposed above the reactor vessel allowing assemblies to be extracted from the core, transferred and placed in a region of the reactor vessel at which a ramp, termed the "primary ramp", opens for transporting assemblies. This primary ramp allows transport of assemblies from the reactor vessel to a transport apparatus termed the "loading and unloading lock chamber" in which the assembly can be transferred to another ramp termed the "secondary ramp" which communicates with a region for storing assemblies or with a region for loading these assemblies into means for transport or temporary storage.
The assemblies are moved inside the primary and secondary ramps by traction apparatuses such as hoists.
The loading and unloading lock chamber contains a movable ramp fixed to a revolving member allowing it to be rotated under the action of drive means disposed at the upper part of the lock chamber so that it moves by pivoting between a position in which the movable ramp forms an extension of the primary ramp and a position in which this movable ramp forms an extension of the secondary ramp.
The traction apparatus is generally fixed on the revolving member and therefore follows the rotary movements of the movable ramp.
Other apparatuses for moving the assemblies inside the primary and secondary ramps can also be fixed to these ramps to move the assemblies during part of their transportation between the vessel and the loading and unloading lock chamber or between the loading and unloading lock chamber and the storage region.
Some loading and unloading lock chambers have two movable ramps the simultaneous rotation allows each of these ramps to be moved from a position in communication with the primary ramp to a position in which it is in communication with the secondary ramp, and vice versa.
These assemblies can thus be moved from the primary ramp to the secondary ramp when the assemblies have been brought inside the movable ramp by traction apparatuses.
The loading and unloading lock chamber and the transfer apparatuses associated with it rest on the structure of the reactor on which the vessel and the attached equipment of the reactor are fixed.
The lock chamber takes up a great deal of space and the cost of constructing this element constitutes a relatively large part of the cost of constructing the reactor.
Also, the lock chamber is in operation only at great intervals corresponding to periods when the reactor is halted for reloading with fuel. In practice, the lock chamber serves not only to unload irradiated assemblies but also to reload the reactor with new assemblies, moving of these assemblies being carried out from the storage region to the reactor core by means of secondary and primary ramps.
The mechanical apparatuses of the loading and unloading lock chamber are therefore brought into use at widely spaced time intervals, so that those mechanical elements which have been at rest for a very long time cannot be relied on to work properly.
There is a current tendency in the construction of fast neutron nuclear reactors to group these reactors, for example in units of two or four, on one and the same site so as to reduce some of the capital cost by shared use of some elements of these reactors.
The use of a single loading and unloading lock-chamber for a group of several reactors has never been thought of before, however.